In optical transport networks having a high range, optical fiber amplifiers whose amplifying fibers are doped with ions of an element originating from the group of rare earths are usually used for signal amplification. Fiber amplifiers doped with erbium ions (“erbium doped fiber amplifiers”, abbreviated to EDFAs) are predominantly used commercially. Such an EDFA has, besides the input for the data signal, an optical pump source, e.g. a laser diode, the output signal of which is coupled into the fiber doped with erbium ions. The optical data signal guided in the doped fiber is amplified by stimulated emission of photons. The EDFAs generally comprise a plurality of amplifier stages. Hereinafter, amplifier stage denotes in each case that part of an EDFA which contains precisely one continuous erbium doped fiber arranged between passive components. Hereinafter, amplifiers which are split into a plurality of amplifier groups are considered, where an amplifier group can comprise either a single amplifier stage or a plurality of amplifier stages.
In order to exhaust the capacity of optical transmission fibers, the data signals are transmitted in individual transmission channels that are often combined by means of the technique of wavelength division multiplexing (abbreviated to WDM). Transmission of WDM signals with up to 80 channels at data rates of up to 40 Gbit/s is possible nowadays by means of the WDM technique. The number of channels varies depending on capacity utilization and transport volume of the transmission system. If channels are switched in and out in the transmission system or coupled in and out at branching points, then this gives rise to abrupt changes in the aggregate signal power in the transmission system. Said changes can lead to bit errors and also to damage at the optical receivers because the latter can operate without any errors only for a limited input power range.
If such abrupt changes in the signal power are present at the input of an optical amplifier, then the pump power of the amplifier has to be adapted rapidly to these power fluctuations of the input signal in order to avoid large jumps in the powers of the channels that are not involved in the switching operation. The output power of an optical amplifier depends on the gain thereof. The amplifier gain is determined by the pump wavelength and pump power in addition to material parameters. Furthermore, the amplifier gain is determined by the input power upon reaching the maximum possible output power (saturation). If the gain remains constant, the power of the channels which are not involved in the switching operation does not change since they are always amplified to the same extent. Therefore, in the design of an optical fiber amplifier, it is always of importance to obtain an amplifier gain that is as constant as possible even in the event of large power jumps at the amplifier input. This is achieved by means of gain regulations. The latter are usually output power regulations in conjunction with an amplified signal, derived from the input signal, as desired value. Methods for regulating the amplifier gain or the amplifier output power are known in many cases from the prior art. Regulating devices supplemented by a control, a so-called feedforward control, are normally used. In the regulating circuit and the feedforward control chain, the optical pump forms the actuating element and the pump power accordingly corresponds to the manipulated variable.
Signal delays are unavoidable in the overall arrangements of optical amplifiers. During signal amplification, in EDFAs in particular, delays of the optical signal occur just as a result of the propagation time in the optical fiber. Said delays amount to approximately 0.3 to 0.6 μs. Furthermore, delays also arise as a result of the physical operation of amplification. When a pump source of 980 nm is used, the electrons of the doping element erbium, during the pumping operation, are initially raised to a first, higher atomic energy level, from which they first relax in a non-radioactive transition to a metastable intermediate level before falling back to the atomic ground level with emission of photons. In addition to these delays of the optical signal, delays of the electrical signal also occur within the regulating device due to the individual structural elements thereof. These include for example delays during the detection and optoelectrical conversion of the input and output signals, delays at the actuators of the pump device and during the signal processing, which can be effected in analog or digital fashion. All these factors adversely influence the regulating behavior, that is to say that the dynamic properties of the regulating device do not lead to an optimum system response. Thus, during the transition recovery time of the regulating device, undesirable transients occur in the amplifier gain, which are manifested in the form of overshoots or undershoots in the output power of the amplifier and in undesirable gain changes.
If a plurality of single-stage amplifiers are cascade-connected in order to obtain higher ranges, then an amplifier cascade arises. Overshoots and undershoots in the output power of the amplifier can accumulate in this case. Small deviations in the gain of individual amplifiers lead to large deviations in the gain at the output of the amplifier cascade. In addition, the abovementioned optical and electrical signal delays make it more difficult to exactly regulate the amplifier gain at the output of the amplifier cascade. Considerable delays of the optical signal of an order of magnitude of 100 μs occur due to the signal propagation time if dispersion compensating fibers (abbreviated to DCFs) are connected between individual amplifier stages.
An earlier German patent application bearing the application number 10 2004 052 883.7 discloses a solution for the compensation of gain fluctuations of a multistage optical amplifier. In the event of a power jump in the input power, the pump power of the first amplifier stage is adapted, the change in the input power that is to be expected at a downstream second amplifier stage is determined and a new pump power for the second pump device is calculated depending on this. In this case, the new pump power is set at the beginning of a predetermined lead time before the arrival of the power jump at the input of the second amplifier stage. One disadvantage of this solution is that the effect of the regulation commences prematurely, and that the gain deviations produced as a result, although they are very small, have a disadvantageous effect in an amplifier cascade. Moreover, the lead time is dependent on the ratio of the optical powers at the input and at the output of the amplifier stages and on the regulator setting. The regulating behavior can be optimized with difficulty under these preconditions.